Video disc playback systems have been proposed and, while they may operate on difference principles, the present invention is uniquely concerned with the optical variety in which information stored on a video disc is derived by scanning the storage track with a reading beam of energy. Information may be stored in such a disc in a variety of ways but the one of particular concern to the present discussion is that in which the storage track may be thought of as a diffracting object so that driving the storage track through the reading beam effects modulation of the beam as required to derive the stored information. In particular, the modulated beam impinges upon photoreceptors which develop the output signal and their operation in that respect is generally the same whether the disc is transmissive to the reading beam or is reflective. It will be convenient in the following discussion to give specific consideration to a transmissive type disc even though the description of the operation is generally applicable to systems functioning in the reflective mode.
In the preparation of a transmissive optical video disc, a master is first prepared under the control of a recording laser beam modulated by the information desired to be stored in the record. Usually the information is carried as frequency modulation of a carrier wave signal employed to modulate the recording laser to establish in the master a pattern of pits and intervening lands of constant width but of variable length to the end that they constitute a spatial representation of the modulated carrier signal. The controlling carrier signal may convey all of the information to be stored, although it is generally necessary to have other signals concurrently control the recording laser so that the stored information may comprise, for example, not only the luminance, chroma and synchronizing componenets of a video program but also audio information and, frequently, one or more pilot signals desirable in optimizing the decoding of the stored information by a player. It has been found desirable, in order to avoid interpenetration of the FM sidebands with signal components in the baseband, to choose the mean frequency of the carrier signal of a value at least twice as high as the highest video frequency modulating component to be reproduced. This enables further information, such as audio subcarriers and pilot signals to be positioned in the baseband for recording purposes without introducing undesirable intermodulation. All such recording techniques are well understood and video players are known for reading the video disc to derive the stored information.
The present invention has to do with the photoreceptor matrix of the video player employed in developing signals representing the program information and other stored information utilized in maintaining desired conditions of tracking registration of the reading beam relative to the storage track of the disc. It is, accordingly, an object of the invention to provide an improved photoreceptor matrix for the optical readout system of a video playback device.
It is another particular object of the invention to provide such a matrix which is especially attractive in separately deriving the carrier signal modulated with program information and relatively low frequency signals such as a pilot signal.